The present invention relates to a double-sided pressure-sensitive adhesive sheet, and specifically it relates to a double-sided pressure-sensitive adhesive sheet which is useful for attachment of a liquid crystal display panel with a backlight unit.
Liquid crystal display devices generally comprise a backlight unit with a light source, and a liquid crystal display panel. For miniature liquid crystal display devices used in cellular phones and the like, the backlight unit and liquid crystal display panel are normally attached by a double-sided pressure-sensitive adhesive sheet. The double-sided pressure-sensitive adhesive sheet is usually designed so as to block light onto the liquid crystal display panel side, because leakage of light onto the liquid crystal display panel side can lead to malfunction of the electronic parts or display irregularities. Providing a black pigment-containing black layer on the double-sided pressure-sensitive adhesive sheet is a commonly known means for blocking light. At the same time, double-sided pressure-sensitive adhesive sheets are preferably designed so as to be reflective onto the backlight unit side, because the light reaching the pressure-sensitive adhesive sheet can be returned to the backlight unit for reuse. A reflective layer such as a metal vapor deposition layer is a known means for conferring reflectivity to a pressure-sensitive adhesive sheet.
A sufficient light-blocking property cannot be achieved when defects such as pinholes are present in the light-blocking black layer. It is generally thought that pinholes are formed such as by foreign matter adhering to the base film. Although formation of pinholes can be prevented by creating a clean environment during formation of the black layer, it is costly to maintain a stringently clean environment. The problem of pinholes is eliminated if the thickness of the black layer is at least 10 μm, but a thick black layer increases the overall thickness of the pressure-sensitive adhesive sheet. If the thickness of the black layer is no more than 10 μm, a pinhole once formed cannot be completely covered up even by use of repeated printings to the above mentioned thickness. Thinner pressure-sensitive adhesive sheets are desired for the lowering thicknesses of liquid crystal displays for cellular phones, but it is difficult to manufacture a thin pressure-sensitive adhesive sheet having light-blocking property onto the side of liquid crystal display panel.
In consideration of this demand, Japanese Unexamined Patent Publication No. 2002-23663 discloses a pressure-sensitive adhesive sheet obtained by laminating in the following order a separator, a pressure-sensitive adhesive layer and a base sheet with at least two metal vapor deposition layers. This pressure-sensitive adhesive sheet is designed with at least two metal vapor deposition layers in order to essentially avoid defects in the light-blocking property due to pinholes or flaws in one metal vapor deposition layer. Also, Japanese Unexamined Patent Publication No. 2002-350612 discloses a plastic film having at least two metal vapor deposition layers which are used for the same purpose as Japanese Unexamined Patent Publication No. 2002-23663. However, as stated in both Japanese Unexamined Patent Publication No. 2002-23663 and Japanese Unexamined Patent Publication No. 2002-350612, the metal vapor deposition layers do not always exhibit adequate light-blocking performance, and often have pinholes. Consequently, when light from the backlight side is not blocked or passes through pinholes, etc. to penetrate the first metal vapor deposition layer, the light is then reflected by the second metal vapor deposition layer. It is thus reflected between the first metal vapor deposition layer and second metal vapor deposition layer, and part of the light fails to be blocked by the second metal vapor deposition layer as it passes through or penetrates pinholes, etc., and thus results in light leakage.
Japanese Unexamined Patent Publication No. 2002-235053 discloses a light-blocking pressure-sensitive adhesive sheet provided with a pressure-sensitive adhesive layer containing a black coloring agent on one or both sides of a base film with low light transmittance. While the pressure-sensitive adhesive sheet and plastic film of the aforementioned Japanese Unexamined Patent Publication No. 2002-23663 and Japanese Unexamined Patent Publication No. 2002-350612 are provided with two or more metal vapor deposition layers to essentially avoid defects in the light-blocking property of one metal vapor deposition layer, the primary object of the pressure-sensitive adhesive sheet disclosed in Japanese Unexamined Patent Publication No. 2002-235053 is to color the pressure-sensitive adhesive layer in order to block light reflected from the pressure-sensitive adhesive cross-section. When the pressure-sensitive adhesive sheet is used, for example, for attachment of a backlight unit with a liquid crystal display panel, it is first punched out to a suitable shape. With such a structure, the pressure-sensitive adhesive seeps out during the punching and can adversely affect the reflection performance of the pressure-sensitive sheet.